1. Field of the Invention
This invention relates to a method of hydrogenating hydrocarbon oil and to a catalyst to be used for such a method and, more particularly, it relates to a method of hydrogenating aromatic hydrocarbons contained in hydrocarbon oil and to a catalyst suitably used for such a method that is highly resistant to sulfur and nitrogen compounds, shows a high hydrogenating activity with a relatively low rate of hydrocracking and has a long service life as catalyst.
2. Background Art
Unsaturated hydrocarbons such as aromatic hydrocarbons contained in hydrocarbon oil are chemically highly reactive and can be easily subjected to various reactions such as oxidation to consequently degrade the hydrocarbon oil with time when compared with saturated hydrocarbons contained in hydrocarbon oil. In the case of diesel gas oil, unsaturated hydrocarbons, aromatic hydrocarbons in particular, contained therein can give rise not only to degradation of the oil in terms of coloring in the course of time, but also to remarkably reduction of the cetane value.
Additionally, in recent years, it has been shown that particulate substances in the exhaust gas of diesel engines can be considerably removed by suppressing the aromatic hydrocarbon content in diesel gas oil and, as a matter of fact, reduction of the aromatic hydrocarbon content in diesel gas oil has been strongly demanded from the viewpoint of environment protection.
There have been reports that polycyclic aromatic hydrocarbons having three or more than three aromatic rings in the molecule provide particularly harmful potential hazards to the environment (e.g., "The Release of the 13th World Petroleum Congress", vol.3, p.97, 1992). Thus, it is an urgent demand to reduce the polycyclic aromatic hydrocarbons contained in hydrocarbon oil and having three or more than three aromatic rings in the molecule. Additionally, an enhanced aromatic hydrocarbon content in kerosene can give rise to the problem of lowering the smoke point thereof and, therefore, the content of unsaturated hydrocarbons, that of aromatic hydrocarbons in particular, of kerosene also has to be reduced in some way or another.
Aromatic hydrocarbons and other unsaturated hydrocarbons can be converted into saturated hydrocarbons by hydrogenating them in the presence of a hydrogenating catalyst, although, while unsaturated hydrocarbons other than aromatic hydrocarbons can be easily hydrogenated, aromatic hydrocarbons are less apt to be end this property on the part of aromatic hydrocarbon provides a serious problem to be solved.
Of known hydrogenating catalysts, those realized by causing a stable carrier such as alumina to carry one or more than one of the VIII-group noble metals in the periodic table are promising because they generally have hydrogenating activity. However, such catalysts are typically accompanied by the drawback that they can be poisoned to quickly become poorly active by sulfur compounds and/or nitrogen compounds contained in hydrocarbon oil.
In order to alleviate this drawback, there have been attempts of hydrogenation using a catalyst having a carrier that contains zeolite. However, zeolite is a catalytic substance that is highly active in hydrocracking reactions and therefore a hydrogenation reaction for the purpose of the present invention is inevitably accompanied by a hydrocracking reaction if a catalyst containing zeolite is used.
In the process of hydrogenating the kerosene and gas oil fractions of hydrocarbon oil, hydrocracking reactions have to be suppressed because such reactions can reduce the yield of the desired kerosene or gas oil fractions.
Japanese Patent Application Laid-Open No. 64-66292 discloses a hydrogenation method using a catalyst containing Y-type zeolite having a unit lattice length of 24.20 to 24.30 angstroms and a silica/alumina ratio of 25 or more, said zeolite carrying one or more than one of the VIII-group noble metals in the periodic table.
Japanese Patent Application Laid-Open No. 5-237391 discloses a catalyst containing one or more than one of the VIII-group noble metals in the periodic table carried by zeolite obtained by treating Y-type zeolite having a unit lattice length of less than 24.65 angstroms and a silica/alumina ratio of more than 5, said zeolite containing less than 0.3 wt % of one or more than one alkali or alkaline earth metals, by contacting with an aqueous solution of alkali or alkaline earth metals so as to raise the alkali or alkaline earth metal content of the zeolite by at least one and a half times greater than the content before treating, and also discloses a hydrogenation method using such a catalyst.
The above known methods are intended to suppress the poisoning of catalyst by sulfur and nitrogen compounds contained in the oil to be treated and also the hydrocracking of unsaturated hydrocarbons.
However, these methods cannot overcome the problem of the poisoning of catalyst by sulfur and nitrogen compounds and hence are not capable of making the catalyst sufficiently active for hydrogenation. Additionally, they cannot satisfactorily alleviate the problem of reduced yield of refined oil due to hydrocracking of the oil to be treated and that of a reduced service life of the catalyst due to the coking phenomenon that appears with hydrocracking.
Therefore, it is an object of the present invention to provide a method of converting aromatic hydrocarbons in hydrocarbon oil containing sulfur and nitrogen compounds into saturated hydrocarbons and a catalyst to be used for such a method that is highly resistant to sulfur and nitrogen compounds and active for hydrogenation and shows a low hydrocracking rate and a long service life.